Field of the Invention
The invention concerns a method for decontamination of natural rubber, allowing elimination of contaminants which may be present in natural rubber. The contaminants are for example vegetables, plastics, mineral granulates and textiles.
Related Art
Natural polyisoprene, also called natural rubber, is obtained from hevea sap, in particular from hevea Brasiliensis. The harvested product must be cleaned, homogenised, coagulated and dried. Coagulation may take place by so-called natural coagulation or artificial coagulation, for example chemically by the addition of acid or salts. Coagulation may occur before the steps of cleaning and homogenisation, or afterwards. The method for processing natural rubber conventionally comprises steps of elimination of contaminants:                primary decontamination intended to eliminate coarse objects, primarily by washing and crushing for coagulated natural rubber or by filtration for uncoagulated natural rubber;        secondary decontamination intended to eliminate finer objects, traditionally performed by passage through creping machines.        
Creping machines are machines with two rotating grooved cylinders between which the rubber passes. This operation may be repeated several times until a sheet is obtained known as “crepe”. Finally, the dry and decontaminated natural rubber is usually provided in the form of cakes or bales.
When the rubber is intended for tire preparation, it is essential that it contains as few contaminants as possible. In general, dry natural rubbers are therefore filtered before use in a rubber compound for tires.
An object of the invention is to use a suitable filtration system allowing decontamination of wet natural rubber. This system may in particular replace completely the creping machines conventionally used, thus allowing better control of the process, obtaining a natural rubber free from contaminants of a size larger than that of the filter mesh, and a gain in the volume occupied.
The filtration of the dry natural rubber treated with a stabiliser is described in patent EP 0 613 924. This patent describes in particular the stabilisation of dry natural rubber with the stabiliser in a heated extruder, with filtration at the extruder outlet by means of a filter. The inventors have found that smooth sheath extruders, as described in this application, cannot be used for natural rubber with a humidity level greater than 12%: the natural rubber slides on the wall of the extruder and cannot advance through the extruder to allow passage of this natural rubber through the filter.
Patent FR 2 320 135 concerns the use of wet natural rubber in an extruder for homogenising the natural rubber, instead of creping machines. This patent claims an extruder with a screw with conical hub, with a progressive pitch and interrupted threads. The wet natural rubber emerges through orifices positioned on the sheaths at the end of the machine. The wet natural rubber does not emerge through a die plate at the end of the screw, as with a conventional extruder. The sheath is equipped with holes allowing evacuation of serum from the extruder. This system cannot be adapted for filtration. Furthermore, the evacuation of serum through holes in the extruder sheath necessitates considerable cleaning and maintenance.
Examples of extruders which may be used in the field of rubber compounds are described in applications EP 492 425, WO99/56938, DE 10 2013 106337.
Application JP 2004/195675 describes an extruder comprising a grooved sheath allowing limitation of heating of the melted plastic. The grooves may have variable forms and dimensions.